1. Field of the Invention
The present invention relates to a method and apparatus for polishing a substrate, and more particularly to a method and apparatus for polishing a substrate such as a semiconductor wafer, a glass substrate, or a liquid crystal display to a flat mirror finish, and then cleaning a polished substrate that requires a high degree of cleanliness.
2. Description of the Related Art
As semiconductor devices have become more highly integrated in recently years, circuit interconnections on semiconductor substrates become finer and the distances between those circuit interconnections have become smaller. One of the processes available for forming such circuit interconnections is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 xcexcm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
Further, if a particle greater than the distance between interconnections exists on a semiconductor substrate, then the particle will short-circuit interconnections on the semiconductor substrate. Therefore, any undesirable particles on the semiconductor substrate have to be sufficiently smaller than the distance between interconnections on the semiconductor substrate.
Therefore, in processing the semiconductor substrate, it is important to planarize the semiconductor substrate and clean the semiconductor substrate to a high degree of cleanliness. Such a requirement holds true for the processing of other substrates including a glass substrate to be used as a mask, a liquid crystal panel, and so on.
In order to achieve planarization of the semiconductor substrate, it has heretofore been customary to use a polishing apparatus, the process of which is called chemical-mechanical polishing (CMP). As shown in FIGS. 10 and 11, a conventional polishing apparatus comprises a polishing section 10, a load and unload section 22, two transfer robots 24a and 24b, a cleaning section 26 having three cleaning units 26a, 26b and 26c, and a reversing device 28 as necessary. The transfer robots 24a and 24b may be a mobile type robot which moves along rails shown in FIG. 10, or a stationary type robot having robot hands at forward ends of articulated arms shown in FIG. 11.
As shown in FIG. 12, the polishing section 10 comprises a turntable 12 having a polishing cloth 11 thereon, a top ring 13 for supporting a semiconductor wafer (substrate) W and pressing the semiconductor substrate W against the turntable 12, and a nozzle 14 for supplying an abrasive liquid onto the polishing cloth 11. A transfer device 38 is disposed adjacent to the turntable 12 (FIGS. 10 and 11). The adjacent two sections are partitioned by a partition wall to prevent cross-contamination. Specifically, in order to prevent dirty atmosphere in the polishing section 10 from being scattered into a chamber in which a cleaning process and a subsequent process are performed, air conditioning and pressure regulation in each of the chambers are performed.
In the polishing apparatus having the above structure, the semiconductor substrate W is transported from the load and unload section 22 to the transfer device 38 by the transfer robots 24a and 24b and transferred to the top ring 13 by the transfer device 38. In the polishing section 10, the semiconductor substrate W is held by the lower surface of the top ring 13 and pressed against the polishing cloth 11 providing a polishing surface on the turntable 12. The abrasive liquid Q is supplied from the nozzle 14 onto the polishing cloth 11 and retained on the polishing cloth 11. During operation, the top ring 13 exerts a certain pressure on the turntable 12, and the surface of the semiconductor substrate W held against the polishing cloth 11 is therefore polished in the presence of the abrasive liquid Q between the surface of the semiconductor substrate W and the polishing surface on the polishing cloth 11 by a combination of chemical polishing and mechanical polishing while the top ring 13 and the turntable 12 are rotated. In case of polishing an insulating layer made of an oxide film (SiO2) on the silicon substrate, the abrasive liquid Q contains abrasive particles having a certain diameter suspended in an alkali solution. The semiconductor substrate W which has been polished is transported to the cleaning section 26 in which the semiconductor substrate W is cleaned and dried, and then returned to the load and unload section 22.
The primary cleaning unit 26a in the cleaning section 26 has a plurality of vertical spindles 30 arranged at spaced intervals for supporting the outer circumferential edge of the semiconductor substrate W by their holding grooves formed at the upper end portions of the spindles 30 and rotating the semiconductor substrate in a horizontal plane at a relatively low rotational speed, and a pair of cleaning members comprising a roller type or a pencil type sponge which can be brought in contact with the semiconductor substrate W or out of contact with the semiconductor substrate W. The primary cleaning unit 26a is a low rotational speed type cleaning unit. The secondary and tertiary cleaning units 26b and 26c have a rotating table 36 comprising a rotating shaft 32 and a plurality of arms 34 which extend radially outwardly from the rotating shaft 32 and hold the outer circumferential edge of the semiconductor substrate W. The secondary and tertiary cleaning units 26b and 26c are a high rotational speed type cleaning unit. In each of the primary, secondary and tertiary cleaning units 26a, 26b and 26c, there are provided a nozzle for supplying a cleaning liquid to the surface of the semiconductor substrate W, a cover for preventing the cleaning liquid from being scattered around, and a ventilating equipment for creating down draft (descending air current) to prevent mist from being scattered around.
The cleaning processes for cleaning the semiconductor substrate which has been polished are carried out in the following manner:
In the primary cleaning unit 26a, while the semiconductor substrate W is held and rotated by the spindles 30, a scrubbing cleaning is performed by allowing the cleaning members to scrub the upper and lower surfaces of the semiconductor substrate W while supplying a cleaning liquid. In this scrubbing cleaning process, the first cleaning liquid having substantially the same pH as the abrasive liquid used in the polishing process is used to prevent particles from being aggregated due to so-called pH shock. To be more specific, if the abrasive liquid is rapidly diluted with pure water to lower the pH of the abrasive liquid, then the abrasive particles become unstable, to thus aggregate secondary particles to form larger aggregates. In this specification, pH shock is defined as a rapid change of a pH. Therefore, in order to prevent pH shock from occurring, the first cleaning liquid having substantially the same pH as the abrasive liquid is used in the scrubbing cleaning process. For example, in case of polishing a SiO2 layer, aqueous ammonia is used as the first cleaning liquid, and after the particles are removed from the surfaces of the semiconductor substrate W, the surfaces of the semiconductor substrate are shifted from alkali to neutrality by supplying a neutral cleaning liquid such as pure water thereto. Thereafter, the semiconductor substrate W is transferred to the secondary cleaning unit 26b. 
In the secondary cleaning unit 26b, in order to remove metal ions attached to the semiconductor substrate W, an acid chemical is normally supplied to the surfaces of the semiconductor substrate W from the nozzle to cause etching (chemical cleaning) of the surfaces of the semiconductor substrate, and then a neutral cleaning liquid such as pure water is supplied to cause the surfaces of the semiconductor substrate to be returned to neutrality. Thereafter, the semiconductor substrate W is transferred to the tertiary cleaning unit 26c having a drying function. In the tertiary cleaning unit 26c, pure water is supplied to perform a final cleaning of the semiconductor substrate W, and then the semiconductor substrate W is rotated at a high rotational speed while blowing a clean inert gas against the surfaces of the semiconductor substrate, thereby drying the semiconductor substrate W. After the semiconductor substrate W is cleaned and dried, the semiconductor substrate W is returned to the load and unload section 22 by a clean hand of the transfer robot 24a or 24b. 
In the above-mentioned polishing apparatus, since etching cleaning is conducted by a dedicated cleaning unit, i.e., the cleaning processes are conducted by the three cleaning units 26a, 26b and 26c, the polishing apparatus becomes large, requires a large installation space, and needs a long processing time for the cleaning processes.
Further, the semiconductor substrate W is transported as shown by arrows {circle around (1)} to {circle around (9)} in FIG. 11, and hence the transportation route is extremely complicated. Specifically, the semiconductor substrate is transported from the load and unload section 22 to the reversing device 28 as shown by the arrow {circle around (1)}, transported to the transfer device 38 as shown by the arrow {circle around (2)}, and then moved above the turntable 12 as shown by the arrow {circle around (3)}. Thereafter, the polished semiconductor substrate W is moved to the transfer device 38 as shown by the arrow {circle around (4)}, transported to the primary cleaning unit 26a as shown by the arrow {circle around (5)}, and then transported to the reversing device 28 as shown by the arrow {circle around (6)}. Thereafter, the semiconductor substrate W is transported to the secondary cleaning unit 26b as shown by the arrow {circle around (7)}, and then transferred to the tertiary cleaning unit 26c as shown by the arrow {circle around (8)}. Finally, the semiconductor substrate W is returned to the load and unload section 22 as shown by the arrow {circle around (9)}.
As is apparent from the above, since the transportation route is extremely complicated, the two transfer robots 24a and 24b which perform the transportation of the semiconductor substrate W are required to be controlled, while preventing the transfer robots 24a and 24b from interfering with each other. Thus, an overall system in the polishing apparatus is more complicated, and tends to retard the processing time of the semiconductor substrate. Further, as the number of times in transporting the semiconductor substrate by the robot hands increases, the chance of contamination of the semiconductor substrate increases.
In order to simplify the structure of the cleaning section 26, it is conceivable to conduct the primary cleaning process for removing particles and the secondary cleaning process for etching by the same cleaning unit. However, in this case, alkali chemicals and acid chemicals are employed as the respective cleaning liquids in the same cleaning unit, and hence the lining of a drainage system is deteriorated, salt is deposited, and waste water treatment is complicated. Alternatively, it is conceivable to conduct etching and drying of the semiconductor substrate in the secondary cleaning unit 26b. However, in this case, the semiconductor substrate W is dried in the atmosphere in which mist of the etching liquid remains, and hence the semiconductor substrate is contaminated in the final cleaning process.
It is therefore an object of the present invention to provide a method and apparatus for polishing a substrate which can reduce the scale of the polishing apparatus, shorten the processing time of the substrate, and produce a substrate having a high degree of cleanliness by performing cleaning processes with a simplified structure.
As the technology for manufacturing semiconductor devices progresses, the characteristics of layers (films) deposited on the semiconductor wafer also progresses. In the case where a semiconductor wafer having a newly developed layer (film) thereon is polished and then cleaned, the multistage cleaning of not less than three-stages is usually conducted at an early stage where the newly developed layer starts to be used in the semiconductor device manufacturing process. However, as the cleaning process progresses, the number of cleaning stages is gradually reduced. Therefore, as the process technology for manufacturing semiconductor devices progresses, the requirement for the apparatus for manufacturing the semiconductor devices changes, and hence an optimum structure of the apparatus must be maintained to meet the progress of the process technology. Therefore, the present invention is directed to provide a method and apparatus for polishing substrates with an optimum structure.
According to one aspect of the present invention, there is provided a polishing apparatus for polishing and then cleaning a substrate, the apparatus comprising: a polishing section having at least one polishing unit for performing primary polishing and secondary polishing of the substrate by pressing the substrate against a polishing surface; a cleaning section for cleaning the substrate which has been polished to remove particles attached to the substrate by scrubbing cleaning, and removing metal ions from the substrate by supplying an etching liquid.
In the secondary polishing, pure water is used as a polishing liquid, and the substrate is polished at a polishing efficiency lower than the primary polishing in such a manner that the polishing pressure and/or the polishing rate are smaller than those of the primary polishing. Thus, microscratches formed on the polished surface of the substrate in the primary polishing are removed or reduced, and ground-off particles and abrasive particles on the polished surface are removed.
For example, in case of using abrasive liquid containing siliceous material, in secondary polishing, particles on the substrate are efficiently removed by polishing the substrate under the final polishing condition. Thus, the substrate having a trace of particles thereon is transported to the cleaning section, and hence a process for removing particles by alkali can be eliminated, different from to the conventional method. In the cleaning section, etching of the surface of the substrate which is a kind of chemical cleaning using acid can be conducted, and then the substrate is cleaned and dried by a cleaning and drying process. In this manner, cleaning and drying of the substrate can be performed by two cleaning units and two processes, and thus the processing time is shortened and the number of apparatuses or units can be reduced, compared with the conventional apparatus and method. Further, the number of times of transporting the substrate is reduced, and hence the processes can be reduced and the chance of contamination of the substrate can be reduced.
It is desirable that final polishing is performed at a polishing rate of 5 xc3x85 (angstrom)/minute or below. The cleaning liquid or the etching liquid may be supplied to the front and backside surfaces of the substrate. The drying process may be performed by a drying unit different from the cleaning unit or the same cleaning unit. If the drying process is performed by the same cleaning unit, then it is desirable to perform the drying process in a clean atmosphere. As a cleaning liquid, a high performance water including ionic water, ozone water, and hydrogen water may be used.
In a preferred embodiment, primary polishing and secondary polishing are conducted in the same polishing unit. This constitution allows the polishing apparatus to be simplified and to reduce an installation space thereof.
In a preferred embodiment, the polishing section comprises at least two polishing units for performing primary polishing and secondary polishing, respectively. This constitution allows the processes to be simplified and contributes to improving throughput of the substrates.
In a preferred embodiment, the polishing section comprises at least two polishing units for the primary polishing process and the secondary polishing process, and the cleaning section comprises at least two cleaning units for performing different cleaning processes. This constitution allows two-stage cleaning or three-stage cleaning, and a plurality of cleaning processes can be performed by the single apparatus.
For example, in case of using an abrasive liquid containing alumina material, removal of particles in the secondary polishing process cannot be sufficiently performed. In performing the etching process after secondary polishing, scrubbing cleaning is carried out with alkali before the etching process to remove particles sufficiently. In this case, three-stage cleaning should be performed.
According to the polishing apparatus of the present invention, two-stage cleaning and the three-stage cleaning can be selectively performed. If the abrasive liquid and the polishing cloth are replaced in the polishing unit, then an optimum apparatus which is suitable for both the process which uses the abrasive liquid containing siliceous material and the process which uses the abrasive liquid containing alumina material may be constructed. Further, even if the scrubbing process becomes unnecessary due to progress in the polishing technology with use of the abrasive liquid containing alumina material and the cleaning technology after polishing, the present invention offers an optimum structure.
According to another aspect of the present invention, there is also provided a polishing method for polishing and then cleaning a substrate, the method comprising: polishing the substrate primarily by pressing the substrate against a polishing surface; polishing the substrate secondarily at a polishing rate lower than the primary polishing; cleaning the substrate which has been polished to remove particles attached to the substrate by scrubbing cleaning, and removing metal ions from the substrate by supplying an etching liquid; and drying the substrate after removing the metal ions therefrom.
In a preferred embodiment, the etching liquid comprises acid aqueous solution containing hydrofluoric acid.
According to another aspect of the present invention, a cleaning apparatus for cleaning a substrate which has been polished comprises: at least two primary cleaning units having the same cleaning function, each for primarily cleaning the substrate which has been polished; and a common secondary cleaning unit for secondarily cleaning the substrate which has been cleaned by the primary cleaning units.
With the above arrangement, the primary cleaning of the substrates is conducted in parallel in the primary cleaning units simultaneously or with a certain time lag, and then secondary cleaning of the substrates which have been primarily cleaned is conducted in a common secondary cleaning unit. This arrangement is desirable for such a case where primary cleaning requires a time longer than secondary cleaning.
According to another aspect of the present invention, a cleaning apparatus for cleaning a substrate which has been polished comprises: at least three cleaning units for cleaning the substrate which has been polished; and a transfer robot for transferring the substrate between at least two of the cleaning units; wherein the substrate is cleaned by selective cleaning processes conducted by any two or three cleaning units selected from the at least three cleaning units.
With the above arrangement, any two or three cleaning units are selected from at least three cleaning units to conduct selective cleaning processes of the substrates. The selective cleaning processes include two-stage cleaning of the substrate or three-stage cleaning of the substrate.
According to another aspect of the present invention, a polishing apparatus for polishing and then cleaning a substrate comprises: a plurality of polishing units for polishing the substrate; and a plurality of cleaning units for cleaning the substrate which has been polished; wherein the substrates are processed by different processes through a plurality of polishing-cleaning routes, each including at least one selected from the polishing units and at least one selected from the cleaning units.
With the above arrangement, at least one selected from a plurality of polishing units and at least one selected from a plurality of cleaning units can be combined to construct a polishing-cleaning route, and the number of polishing-cleaning routes can be freely selected by setting the number of polishing units and cleaning units appropriately, and hence a plurality of different processes can be selectively conducted on the substrates.